The invention relates to the field of acquiring and analysing biological signals, more specifically signals representing blood flow velocity and blood pressure. The invention will be described with reference to the problem of acquiring information concerning the physiological status of the brain from the measurement and analysis of blood flow velocities in the brain arteries and systemic blood pressure. However, the invention may also be used to acquire information concerning the physiological status of (blood flow velocity and blood pressure in) other body parts or organs.
Each time the heart beats, it forces blood out into the arteries of the body. A portion of this blood enters the skull through the carotid arteries to supply oxygen and nutrients to the brain. The pumping of the blood by the heart creates a pulsation of the blood through the arteries. This pulsation of the blood in the arteries of the brain contributes to the fluid pressure levels in the brain. The high level of brain metabolism requires a constant supply of oxygen and nutrients regardless of possible shifts in systemic blood pressure occurring due to variations in heart rhythm or respiratory function. The flow of blood to the brain is automatically regulated by the body to avoid cerebral hypo- or hyperperfusion. Furthermore, the body maintains arterial blood pressure within safe limits, on the one hand maintaining adequate pressure levels for cerebral perfusion, on the other preventing high pressure levels to reach into the cerebral capillaries with a risk of fluid leakage or haemorrhage. Finally, other regulatory processes in the body operate to keep the level of the intracranial pressure within safe limits. Some of these other regulatory processes include the formation and absorption of cerebro-spinal fluid, maintenance of carbon dioxide levels in the blood, etc. When the brain is subjected to head trauma, internal bleeding, brain tumors or other abnormal conditions, the intracranial pressure may rise to dangerous levels impeding cerebral perfusion. If the body's regulatory processes are not able to control the increased intracranial pressure, then death may result.
In 1982, transcranial Doppler ultrasound (TCD) was introduced, a non-invasive technique for the measurement of flow velocities in intracranial arteries by means of ultrasound signals transmitted through bone (see for example Peters P, Datta K: Middle cerebral artery blood flow velocity studied during quiet breathing, reflects hypercapnic breathing in man. In: Modelling and control of ventilation, Semple S J G, Adams L and Whipp B J (eds). Plenum Press New York 1995; 293-295). TCD has since been used for a great number of medical indications, amongst which the measurement of vasospasm in subarachnoid hemorrhage, the monitoring of intracranial flow velocities during carotid endarterectomy (CEA), the determination of collateral flow over the circle of Willis, the investigation of cerebral autoregulation, etc. CEA is a surgical procedure in which fatty deposits are removed from one of the carotid arteries. Carotid artery problems become more common as people age. The disease process that causes the formation of fat and other material on the artery walls is called atherosclerosis, popularly known as “hardening of the arteries.” The fatty deposit is called plaque; the narrowing of the artery is called stenosis. The degree of stenosis is usually expressed as a percentage of the normal vessel diameter at the site of measurement.
TCD technology allows to analyse blood flow velocity with a high temporal resolution, which makes it a much more attractive method for bedside monitoring compared to other diagnostic brain imaging techniques such as FMRI (functional magnetic resonance imaging) or PET (positron emission tomography) analysis.
Although TCD is an elegant diagnostic tool as well as a convenient monitoring procedure, its widespread use has been hampered by the often difficult interpretation of the signal. For instance, when interpreting flow velocity measurements over the middle cerebral artery (MCA), often several confounding factors need to be taken into account, such as the angle of insonation (usually unknown in TCD); fluctuations in heart beat frequency; condition of the carotid arteries and/or possible changes in MCA vessel diameter; cerebrovascular resistance (CVR); respiratory function (blood level of carbon dioxide); the arterial blood pressure (ABP); the intracranial pressure (ICP); variations in anatomy of the circle of Willis; mental status and turbulence due to blood flow derived from collateral vessels such as, for instance, the anterior communicating artery.
Together, the factors mentioned above give rise to a considerable inter- and intra-individual variation in flow velocity measurement. Thus, to improve the relevance of TCD-monitoring, a reliable interpretation of the TCD signal is required.